There are many types of vehicle security systems or vehicle anti-theft systems in use today for preventing unauthorized entry into, and operation of, a vehicle. A system such as this usually includes some means for disabling the vehicle when the system is armed or rendered operative and some kind of an alarm which sounds when attempts are made to enter the vehicle or to lift the vehicle's hood or to tamper with the security system. In systems designed for incorporation into vehicles with power door locks, the system may also include a module for automatically actuating the door locks when the vehicle is armed or rendered operative.
Some vehicle security systems are so-called active systems in the sense that the system is armed and disarmed by actuation of a key switch outside the vehicle. Some contemporary active systems use coded radio frequency (rf) transmitters and receivers to arm and disarm the system. An example of this type of arrangement is disclosed in U.S. Pat. No. 3,703,714.
Other systems are denominated passive systems because they become armed or set automatically after the operator leaves the vehicle. For example, some passive systems become armed a selected time after the ignition has been turned off. Usually such systems require the installation of a conductor from the security system to the ignition circuitry to sense whether or not the ignition is turned on. Other systems are set automatically after all of the vehicle doors have been closed. The trouble with these prior passive systems is that they fail to take into consideration whether or not there are any passengers in the vehicle at the time the system is armed. Obviously, if a person or pet happens to be in the car when the system becomes armed, any motion of that person or pet would set off the alarm. Also, if the system includes an automatic door lock feature, the arming of the system may often result in the locking of the doors, making it more difficult for a less competent passenger such as a small child or an aged or infirmed adult to escape from the vehicle in an emergency situation.